loadChunk: zero extent

[ax3l]

Describe the bug
Seen with #952: it should be allowed to load a zero-size extent.

corrupted double-linked list

This should produce an empty shared_ptr / zero-sized array (note that for numpy, only the 0:0 selected axes are set to zero-extent in the returned array).

To Reproduce

... having a hard time to write a reproducer outside of the location linked above..
https://github.com/ax3l/openPMD-api/tree/topic-zeroExtendRead

openPMD-api/src/binding/python/openpmd_api/DaskArray.py

Lines 39 to 46 in f5dc8e2

	# FIXME: implement handling of zero-slices in Record_Component
	# https://github.com/openPMD/openPMD-api/issues/957
	all_zero = True
	for s in slices:
	if s != np.s_[0:0]:
	all_zero = False
	if all_zero:
	return np.array([], dtype=self.dtype)

Python:

import openpmd_api

# ...
data = rc[0:0]
rc.series_flush()

or C++

#include <openPMD/openPMD.hpp>

// ...

Expected behavior
At the moment, this crashes (at least in Python). Instead, it should return a zero-sized variable/array.

Software Environment

• version of openPMD-api: 0.13.2
• installed openPMD-api via: from source
• operating system: Ubuntu 20.04
• machine: local laptop
• name and version of Python implementation: CPython 3.9.2
• version of HDF5: 1.10.6
• version of ADIOS1: N/A
• version of ADIOS2: 2.7.1
• name and version of MPI: N/A

Additional context
Seen with #952: This is used by Dask to "probe" the data set on initial access.